1. Field of the Invention
The present invention relates to a recorder which records an image on a record medium by a light beam.
2. Related Background Art
A light scan system which uses a laser as a light source and uses a rotating polygon mirror or a vibrating mirror has been widely used in a facsimile machine, a display device and a printer because of its wide scan angle and a small color dispersion. The rotating polygon mirror is frequently used in a high speed scan apparatus. In such a scan apparatus, the scan pitch of scan lines may not be uniform due to an inclination of the rotating polygon mirror, variation of drum rotation speed and vibration of the rotating polygon mirror.
FIG. 24 shows a gray scale of gray level outputs produced by a dither method or a density pattern method. Eight levels a-h of outputs are extractively or selectively shown. A recorder which records at a high image quality has 64 levels of outputs, for example (Electro-photographing by the dither method and the density pattern method is disclosed in the First Non-Impact Printing Technique Symposium Papers, 1984, page 88 by Kawamura, Kitajima and Kadowaki.)
As seen from FIG. 24, the scan pitch nonuniformity is remarkable in the high image density areas f-h. The reason for this is explained below.
FIG. 25A shows a 4.times.4 dither threshold matrix for explanation purposes. The threshold matrix of FIG. 25A is of dot concentration type (fatting type) and produces a pseudo-netpoint pattern as an output image as shown in FIG. 25B for a uniform input data D=1 (highlight area on a screen). Assuming that ideal recording without pitch nonuniformity or vibration is effected, the areas corresponding to the level I of the threshold matrix are blackened and a uniform pattern is produced. (The blocks represent positions of pixels to be recorded.)
If the pitch nonuniformity is included and the upper dot in FIG. 25B is shifted one pixel down and the lower dot is shifted one pixel up, the output at the highlighted area of the image is produced as shown in FIG. 25C in which individual dot densities are preserved.
On the other hand, if the input image data to a shadow area is uniformly "3", an output shown in FIG. 25D is produced and black dots continuously appear. An image density increases by the continuous dots. (area 21 in FIG. 25D)
This is explained with reference to FIGS. 26A and 26B. A distribution of light energy in a section P--P' of FIG. 25B is shown by A, and a distribution of blackened toner is shown by A'. When a spatial distance between two dots is large and the light energy distribution in two Gauss beam sections shown by A in FIG. 26A is developed at a development level t, it appears as shown by A'.
On the other hand, when two dots approach each other by the pitch nonuniformity as shown in FIG. 26B, a bottom slope of the combined light energy curve is raised as shown by "30" of B in FIG. 26B. Accordingly, an output B" is produced which has an increased black area compared to an output B' in which two black areas merely approach each other.
It is seen from the above description that the pitch nonuniformity is more prominent in the high image density area than in the intermediate image density area.
Since the pitch nonuniformity significantly affects the print quality, various compensation methods therefor have been proposed. All of those methods use complex optical systems and lead to an increase in cost and reduction of reliability due to the complex structure required for their implementation.